The present invention relates generally to radio communication systems and, in particular, to built-in antennas incorporated into portable terminals and having a wide bandwidth to facilitate operation of the portable terminals within different frequency bands.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing the next generation technology, e.g., time division multiple access (TDMA) or code division multiple access (CDMA). Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure.
To provide an acceptable level of equipment compatibility, standards have been created in various regions of the world. For example, analog standards such as AMPS (Advanced Mobile Phone System), NMT (Nordic Mobile Telephone) and ETACS and digital standards such as D-AMPS (e.g., as specified in EIA/TIA-IS-54-B and IS-136) and GSM (Global System for Mobile Communications adopted by ETSI) have been promulgated to standardize design criteria for radio communication systems. Once created, these standards tend to be reused in the same or similar form, to specify additional systems. For example, in addition to the original GSM system, there exists the DCS1800 (specified by ETSI) and PCS1900 (specified by JTC in J-STD-007), both of which are based on GSM. A recent evolution in cellular communication services involves the adoption of additional frequency bands for use in handling mobile communications, e.g., for Personal Communication Services (PCS) services. Taking the U.S. as an example, the Cellular hyperband is assigned two frequency bands (commonly referred to as the A frequency band and the B frequency band) for carrying and controlling communications in the 800 MHZ region. The PCS hyperband, on the other hand, is specified in the United States to include six different frequency bands (A, B, C, D, E and F) in the 1900 MHZ region. Thus, eight frequency bands are now available in any given service area of the U.S. to facilitate communication services. Certain standards have been approved for the PCS hyperband (e.g., PCS1900 (J-STD-007)), while others have been approved for the Cellular hyperband (e.g., D-AMPS (IS-136)). Other frequency bands in which these devices will be operating include GPS (operating in the 1.5 GHz range) and UMTS (operating in the 2.0 GHz range).
Each one of the frequency bands specified for the Cellular and PCS hyperbands is allocated a plurality of traffic channels and at least one access or control channel. The control channel is used to control or supervise the operation of mobile stations by means of information transmitted to and received from the mobile stations. Such information may include incoming call signals, outgoing call signals, page signals, page response signals, location registration signals, voice channel assignments, maintenance instructions, hand-off, and cell selection or reselection instructions as a mobile station travels out of the radio coverage of one cell and into the radio coverage of another cell. The control and voice channels may operate using either analog modulation or digital modulation.
The signals transmitted by a base station in the downlink over the traffic and control channels are received by mobile or portable terminals, each of which have at least one antenna. Historically, portable terminals have employed a number of different types of antennas to receive and transmit signals over the air interface. For example, monopole antennas mounted perpendicularly to a conducting surface have been found to provide good radiation characteristics, desirable drive point impedances and relatively simple construction. Monopole antennas can be created in various physical forms. For example, rod or whip antennas have frequently been used in conjunction with portable terminals. For high frequency applications where an antenna's length is to be minimized, another choice is the helical antenna.
In addition, mobile terminal manufacturers encounter a constant demand for smaller and smaller terminals. This demand for miniaturization is combined with desire for additional functionality such as having the ability to use the terminal at different frequency bands and different cellular systems.
It is commercially desirable to offer portable terminals which are capable of operating in widely different frequency bands, e.g., bands located in the 1500 MHZ, 1800 MHZ, 1900 MHZ, 2.0 GHz and 2.45 GHz regions. Accordingly, antennas which provide adequate gain and bandwidth in a plurality of these frequency bands will need to be employed in portable terminals. Several attempts have been made to create such antennas.
Japanese patent no. 6-37531 discloses a helix which contains an inner parasitic metal rod. In this patent, the antenna can be tuned to dual resonant frequencies by adjusting the position of the metal rod. Unfortunately, the bandwidth for this design is too narrow for use in cellular communications.
Dual-band, printed, monopole antennas are known in which dual resonance is achieve by the addition of a parasitic strip in close proximity to a printed monopole antenna. While such an antenna has enough bandwidth for cellular communications, it requires the addition of a parasitic strip. Moteco AB in Sweden has designed a coil matching dual-band whip antenna and coil antenna, in which dual resonance is achieved by adjusting the coil matching component (1/4.lambda. for 900 MHZ and 1/2.lambda. for 1800 MHZ). This antenna has relatively good bandwidth and radiation performances and a length in the order of 40 mm. A non-uniform helical dual-band antenna which is relatively small in size is disclosed in copending, commonly assigned U.S. patent application Ser. No. 08/725,507, entitled "Multiple Band Non-Uniform Helical Antennas."
Conventional built-in antennas currently in use in mobile phones include microstrip antennas and planar inverted-F antennas. Microstrip antennas are small in size and light in weight. The planar inverted-F antenna (PIFA) has already been implemented in a mobile phone handset, as described by K. Qassim, "Inverted-F Antenna for Portable Handsets", IEE Colloqium on Microwave Filters and Antennas for Personal Communication Systems, pp.3/1-3/6, February 1994, London, UK. More recently, Lai et al. have published a description of a meandering inverted-F antenna (WO 96/27219). This antenna has a size which is about 40% of that of the conventional PIFA antenna.
However, as mobile phones become smaller and smaller, both conventional microstrip patch and PIFA antennas are still too large to fit future phone chassis. In copending, commonly assigned U.S. patent application No. 09/112,366, entitled "Miniature Printed Spiral Antenna for Mobile Terminals", a printed spiral built-in antenna with a matching post was proposed. The size of the antenna was reduced to 20-30% of the conventional PIFA antenna, which is less than 1/10.sup.th of a wavelength, in order to make it suitable for future mobile phones.
In addition to a reduced antenna size, next generation mobile phones will require the capability to tune to more than one frequency band for cellular, wireless local area network, GPS and diversity. In copending, commonly assigned U.S. patent application Ser. No. 09/112,152, entitled "Twin Spiral Dual Band Antenna", a multiple band, built-in antenna was proposed which is suitable for future mobile phones. The built-in antenna comprises two spiral conductor arms which are of different lengths and capable of being tuned to different frequency bands. In this design, the bandwidth of the antenna is smaller because thin strip lines are used as radiators. In order to increase bandwidth of the antenna, a compensation method is used by introducing a resistor loading technique on the matching bridge. While this approach leads to a wider bandwidth, it also results in a loss of gain. This antenna is designed for use in two frequency bands.
In copending, commonly assigned U.S. patent application Ser. No. 09/212,259, entitled "Printed Multi-Banded Patch Antenna", another new type of dual band patch antenna is disclosed. In contrast to the twin spiral dual band antenna which uses thin strip lines as radiators, the multi-band patch antenna uses patches with slot cutting. The patches are used as radiators and facilitate a wider bandwidth. The multi-band patch antenna is also designed for two frequency bands.
FIG. 1 illustrates the geometry of a conventional PIFA antenna 100. The PIFA antenna includes a radiating element 110, a feeding pin 120 for the radiating element, a ground pin 130 for the radiating element and a printer circuit band (PCB) ground 140. The radiating element 110 is suspended above the PCB ground 140 in such a manner that the PCB ground 140 covers the area under the radiating element 110. This type of antenna, however, has a small bandwidth in the order of 100 MHZ. In order to increase the bandwidth for an antenna of this design, the vertical distance between the radiating element and the PCB ground has to be increased (that is, the height at which the radiating element 110 is placed above the PCB 140 is increased). This, however, is an undesirable modification as the height increase makes the antenna unattractive for small communication devices.
An alternative method for obtaining a greater bandwidth is illustrated by the antenna 200 of FIG. 2 which corresponds to the antenna design of U.S. patent application No. 09/507,673 referred to above. The PCB board 240 of the antenna 200 does not cover the entire area under the radiating element 210. This increases the distance between the radiating element 210 and the PCB ground 240. That is, the radiating element 210 extends out from the edge of the PCB 240. While the design of antenna 200 leads to a greater bandwidth than antenna 100 of FIG. 1, it is not adequate for covering the frequency bands corresponding to DCS, PCS and UMTS.
The antennas described above lack adequate bandwidth to cover, for example, all of the DCS, PCS and UMTS frequency bands. Therefore, there exists a need for a lowprofile, built-in antenna which can be incorporated into portable terminals and which allow the portable terminals to communicate within the different frequency bands.